Police have released CCTV images of four men believed to have been involved in violence during a match between Burnley FC and German club Hannover 96.
Trouble broke out between both sets of fans 38 minutes into the friendly game at Turf Moor, in the David Fishwick stand, on 5 August.
About £9,000 in damage was caused and the game was abandoned at half-time.
Two men have already been arrested in connection with the investigation.
One, aged 21 and a German national, was released with no charge.
The second, aged 23 and from Burnley, was arrested on suspicion of violent disorder and released under investigation.
A 15-year-old boy and a 63-year-old man from Nelson and Barnoldswick voluntarily attended a police station and were also interviewed.
Det Con Anthony Atkins, of Lancashire Police, said: “We would like to speak to the four people in the CCTV images and would urge anybody who recognises them to get in touch as soon as possible.
“Similarly, we would urge the men themselves, if they see this appeal, to contact us.”